1. Field of the Invention
The invention relates to a method for fabricating a transistor, and more particularly, to a method for fabricating a metal gate transistor.
2. Description of the Prior Art
In the field of semiconductor fabrication, the use of polysilicon material is diverse. Having a strong resistance for heat, polysilicon materials are commonly used to fabricate gate electrodes for metal-oxide semiconductor transistors. The gate pattern fabricated by polysilicon materials is also used to form self-aligned source/drain regions as polysilicon readily blocks ions from entering the channel region.
However, devices fabricated by polysilicon still have many drawbacks. In contrast to most metal, polysilicon gates are fabricated by semiconductor materials having high resistance, which causes the polysilicon gate to work under a much lower rate than other metal gates. In order to compensate for slightly lowered rate of performance, a significant amount of silicides is applied during the fabrication of polysilicon processes, such that the performance of the device could be increased to an acceptable level.
Gate electrodes fabricated by polysilicon also causes a depletion effect. In most circumstances, the optimum doping concentration for polysilicon is between about 2×2020/cm3 and 3×1020/cm3. As most gate electrodes have a doping concentration of at least 5×1021/cm3, the limited doping concentration of polysilicon gates often results in a depletion region at the interface between the gate and the gate dielectric layer. This depletion region not only thickens the gate dielectric layer, but also lowers the capacitance of the gate, and ultimately reduces the driving ability of the device. In order to solve this problem, double work function metal gates are used to replace conventional polysilicon to fabricate gate electrodes for MOS transistors.
However, it is well known in the art that the degree of difficulty for fabricating a well-controlled double work function metal is immense as the process often involves complicated integration between NMOS device and PMOS device. The difficulty increases even more as the thickness and materials used in double work function metal gates requires a much more strict demand. Hence, how to successfully fabricate double work function metal gate transistors with lower cost and improved performance has become an important task in this field.